1. Field of the Invention
The present invention relates generally to flow control devices for fluid conveying pipelines and, more specifically, to a fusable check valve made from thermoplastic materials for use as a flow control device in such pipelines.
2. Description of the Prior Art
At the present time, plastic pipe materials, particularly polyolefins such as polyethylene, are replacing iron and steel as conduits for the conveyance of fluids including water and sewage as well as hydrocarbons, natural gas and other chemical and industrial fluids. Such “plastic” pipelines have advantages over metal conduits made of iron and steel since the use of iron and steel in underground applications can lead to corrosion and deterioration of the materials.
In the case of, for example, a natural gas pipeline, polyethylene conduit can provide the advantages of corrosion resistance and durability. Polyethylene has been used in the past to not only fabricate gas pipelines but also in the fabrication of certain types of valves for controlling gas flow through the pipelines. For example, polyethylene ball valves and plug valves are commercially available for installation in gas distribution systems.
U.S. Pat. No. 5,149,054 shows a “plug” valve which is molded from a polymeric material and is utilized as a shut off valve in a gas pipeline.
U.S. Pat. No. 3,526,386 shows a valve with a body which is formed of high density polyethylene.
U.S. Pat. No. 4,348,006 shows a molded polyethylene valve body which houses a “butterfly” valve component.
U.S. Pat. No. 5,154,396 shows a polyethylene plastic valve which is said to be useful in various piping systems in a wide range of environmental conditions.
U.S. Pat. No. 5,639,394 shows a valve assembly having polyethylene valve body which is formed by electrofusing a coupling to pipe fittings.
Despite the above advances in the art, none of the cited references deal specifically with back flow prevention or “check valve” applications. In the discussion which follows, the term “check valve” will mean a valve in the purest sense which is open when upstream pressure appears and passes all flow in the normal direction without resistance but which then closes at the instant of zero flow velocity and remains closed during minor forward pressure surges and resists back pressure without leakage. Whereas other types of valves variably restrict flow volume, the check valve is normally either open or closed and not subject to external regulation. The force to open the valve comes from the upstream pressure of the fluid itself, rather than operator intervention.
Prior art check valves utilizing metal valve components for plastic pipelines have generally required a transition piece followed by the metallic valve body followed, in turn, by a nipple, union and additional transition. This added to the expense of the plastic pipeline and was labor intensive.
There remains a need for an improved check valve for plastic pipelines which overcomes the various deficiencies of the prior art devices.